The present invention relates to motor drive technology, and more particularly, to a motor drive technology of a pulse width modulation (PWM) system.
As PWM drive systems for a brushless motor, a triangular wave slicing system and a peak current detecting system are known. In the triangular wave slicing system, a coil current is made to flow through a detection resistance, and the difference between a voltage generated at the detection resistance and a torque command voltage is output as a slice level by an error amplifier. A triangular wave having a constant period is sliced with the slice level, to determine the time period (ON period) during which the current flows to the coil. In the peak current detecting system, which uses no error amplifier, supply of a current to a coil is halted when the voltage generated at the current detection resistance, through which the coil current flows, reaches the torque command voltage, and a regenerative current mode is started.
FIG. 13 is a block diagram of a conventional motor driver of the peak current detecting method. Referring to FIG. 13, Hall sensors 21A, 21B and 21C detect the position of a rotor of a motor 10 and output the detection results to a position detection circuit 22 as Hall sensor outputs S11, S12 and S13, respectively. The position detection circuit 22 determines position signals S21, S22 and S23 based on the Hall sensor outputs S11, S12 and S13, respectively, and outputs the signals to a phase switch circuit 93. The position signals S21, S22 and S23 are signals obtained by shifting the phase of the Hall sensor outputs S11, S12 and S13 by 30xc2x0.
The phase switch circuit 93 determines the phases of currents to pass according to the position signals S21, S22 and S23. For easy measurement of the phase currents, the phase switch circuit 93 blocks flow of one of three phase currents. A Logic control circuit 95, set upon receipt of a reference pulse PI, controls supply of currents to the motor 10 by changing the level of signals output to the phase switch circuit 93. The reference pulse PI is a periodical pulse.
FIG. 14 is a graph showing changes with time of phase currents for the motor driven by the motor driver of FIG. 13. In FIG. 14, phase currents 11, 12 and 13 in U, V and W phases, respectively, are shown, and currents flowing from drive transistors 1 to 6 toward the motor 10 are considered positive. As is found from FIG. 14, there is always one phase current that becomes zero, and thus there occurs sharp change of any of the phase currents every electrical angle of 60xc2x0.
Assume that the logic control circuit 95 has been set with the reference pulse PI. The phase switch circuit 93 turns ON only the W-phase upper side drive transistor 5 and the U-phase lower side drive transistor 2, for example. In this state, a current flows to a current detection resistance 7 via a W-phase coil 13 and a U-phase coil 11. The magnitude of this current can therefore be detected as the voltage generated at the current detection resistance 7. Since this current flows through the inductive coils, the current gradually increases after the conduction of the drive transistors 2 and 5.
With increase of the current, the voltage generated at the current detection resistance 7 increases, and when it reaches a torque command voltage TI, the level of the output of a comparator 96 changes, causing the logic control circuit 95 to be reset. The reset logic control circuit 95 reverses the level of a signal output to the phase switch circuit 93. On receipt of this signal, the phase switch circuit 93 turns OFF the drive transistor 2.
The time period from the setting of the logic control circuit 95 until the reset thereof corresponds to the on-duty period of switching operation. After the reset of the logic control circuit 95, the current flowing through the coils 11 and 13 still attempts to continue the flow, and this causes a regenerative current to flow through a diode 1D existing between the source and drain of the drive transistor 1. Since the regenerative current does not pass through the current detection resistance 7, the voltage generated at the current detection resistance 7 is zero during the flow of the regenerative current.
The regenerative current gradually decreases. However, upon receipt of the reference pulse PI, the logic control circuit 95 is set again, and the phase switch circuit 93 turns ON the drive transistor 2. This operation is repeated until the phase switch circuit 93 switches the phases of currents to pass. In this way, as a result of the alternate flow of the drive current flowing when the logic control circuit 95 is set and the regenerative current flowing when the logic control circuit 95 is reset, a phase current roughly corresponding to the torque command voltage TI is allowed to flow through a predetermined coil.
FIG. 15 is a graph showing the current detection resistance voltage (motor current detection signal) MC and the V-phase and W-phase currents 12 and 13 at and around time t=tz in FIG. 14, obtained by enlarging the time axis. In FIG. 15, a period T91 is a time period during which a drive current of the U-phase and V-phase currents flows. This drive current flows through the current detection resistance 7. A period T92 is a time period during which the U-phase and V-phase currents flow as a regenerative current. A period T93 is a time period during which a drive current of the U-phase and W-phase currents flows. This drive current flows through the current detection resistance 7. A period T94 is a time period during which the U-phase and W-phase currents flow as a regenerative current.
The conventional motor driver shown in FIG. 13 has the following problem. The phase currents sharply change as shown in FIG. 14. For this reason, when the phase currents are switched, vibration of the motor and generation of electromagnetic noise tend to occur.
To avoid the above problem, the phase currents may be controlled not to change sharply. However, to detect and control a plurality of phase currents, it is necessary to provide current detection resistances in the same number as the number of phases. It is difficult to incorporate the current detection resistances in an integrated circuit. Therefore, as the number of the current detection resistances is greater, the scale of the device is larger and the cost is higher.
In addition, the properties of resistances generally have variations. Therefore, in the case of using current detection resistances for the respective phases, the current detection properties vary every phase. For example, when two phase currents are actually the same in magnitude, the magnitudes of the detected currents may sometimes be different from each other.
An object of the present invention is providing a motor driver capable of controlling a plurality of phase currents not to change sharply, using one current detection resistance, to reduce vibration of the motor and electromagnetic noise.
The present invention is directed to a motor driver having a plurality of output circuits each including an upper side switching element and a lower side switching element connected in series, for supplying a current to a motor from a connection point between the upper side switching element and the lower side switching element of each output circuit. The motor driver includes: a current detection resistance connected in series with the plurality of output circuits in common for detecting a current supplied to the plurality of output circuits; a position detection section for outputting a position signal corresponding to the position of a rotor of the motor; a phase switch circuit for selecting one switching element of one of the plurality of output circuits according to the position signal and turning ON the selected switching element for a time period corresponding to a predetermined electrical angle, and switching lower side switching elements of a plurality of output circuits among the remaining ones of the plurality of output circuits when the selected switching element is an upper side switching element while switching upper side switching elements of a plurality of output circuits among the remaining ones of the plurality of output circuits when the selected switching element is a lower side switching element; and an ON-period control section for generating a switching control signal for controlling the switching operation by the phase switch circuit according to an input torque command signal and a voltage generated at the current detection resistance so that each of a plurality of periods obtained by dividing the time period corresponding to the predetermined electrical angle includes a first period in which a plurality of switching elements among the switching elements to be switched are turned ON and a second period in which one of the plurality of switching elements turned ON in the first period is kept ON, and outputting the generated signal.
According to the invention, there are provided the first period in which a plurality of switching elements are turned ON and the second period in which one of the plurality of switching elements turned ON in the first period is kept in the ON state. Therefore, a plurality of phase currents can be controlled using one current detection resistance. This enables PWM control with no variation in magnitude of the phase currents. In addition, the phase currents are avoided from sharp change, and thus vibration of the motor and electromagnetic noise during the phase switch can be reduced.
In the motor driver described above, preferably, the ON-period control section includes: a torque signal generation circuit for generating a first target signal corresponding to a target value of a current that should flow to the current detection resistance during the first period, according to the torque command signal, and a second target signal corresponding to a target value of a current that should flow to the current detection resistance during the second period, determined according to the torque command signal and the position signal; a comparator for determining whether or not the voltage generated at the current detection resistance exceeds the output of the torque signal generation circuit and outputting the result; and a logic control circuit for generating the switching control signal according to a reference pulse for defining the period of the switching operation and the output of the comparator and outputting the generated signal, wherein the logic control circuit generates the switching control signal so that the first period is terminated when the comparator determines that the voltage generated at the current detection resistance has exceeded the output of the torque signal generation circuit for the first period and that the second period is terminated when the comparator determines that the voltage generated at the current detection resistance has exceeded the output of the torque signal generation circuit for the second period, and outputs the generated signal. With this configuration, a suitable switching control signal can be generated.
Preferably, the logic control circuit includes: a first latch set with the reference pulse and reset with the output of the comparator; a second latch set with the reference pulse; and a logic circuit receiving the output of the first latch and the output of the comparator for supplying the resultant output to the second latch as a reset input, the logic control circuit outputting the outputs of the first latch and the second latch as the switching control signal, wherein the first latch is reset when the output of the comparator indicates that the voltage generated at the current detection resistance has exceeded the first target signal, the logic circuit outputs the output of the comparator when the output of the first latch indicates that the first latch has been reset, and does not output the output of the comparator when the output of the first latch indicates that the first latch has not been reset, and the second latch is reset when the logic circuit outputs the output of the comparator and the output of the comparator indicates that the voltage generated at the current detection resistance has exceeded the second target signal. Having the logic circuit, the operation of the second latch is ensured, and thus malfunction of the motor driver can be reduced.
Preferably, the logic control circuit further includes a delay circuit for outputting the output of the first latch delayed by a predetermined time, wherein the first latch supplies the output to the logic circuit via the delay circuit. With this configuration, malfunction due to noise at the second latch can be reduced.
In the motor driver described above, preferably, the ON-period control section includes: a torque signal generation circuit for outputting a first target signal corresponding to a target value of a current that should flow to the current detection resistance during the first period, according to the torque command signal, and a second target signal corresponding to a target value of a current that should flow to the current detection resistance during the second period, determined according to the torque command signal and the position signal; a first comparator for determining whether or not the voltage generated at the current detection resistance has exceeded the first target signal and outputting the result; a second comparator for determining whether or not the voltage generated at the current detection resistance has exceeded the second target signal and outputting the result; and a logic control circuit for generating the switching control signal according to a reference pulse for defining the period of the switching operation and the outputs of the first and second comparators and outputting the generated signal, wherein the logic control circuit generates the switching control signal so that the first period is terminated when the first comparator determines that the voltage generated at the current detection resistance has exceeded the first target signal and that the second period is terminated when the second comparator determines that the voltage generated at the current detection resistance has exceeded the second target signal, and outputs the generated signal. With this configuration, since the first and second comparators cause no malfunction easily, stable operation is possible.
Preferably, the logic control circuit includes: a first latch set with the reference pulse and reset with the output of the first comparator; a second latch set with the reference pulse; and a logic circuit receiving the output of the first latch and the output of the second comparator for supplying the resultant output to the second latch as a reset input, the logic control circuit outputting the outputs of the first and second latches as the switching control signal, wherein the first latch is reset when the output of the first comparator indicates that the voltage generated at the current detection resistance has exceeded the first target signal, the logic circuit outputs the output of the second comparator when the output of the first latch indicates that the first latch has been reset, and does not output the output of the second comparator when the output of the first latch indicates that the first latch has not been reset, and the second latch is reset when the logic circuit outputs the output of the second comparator and the output of the second comparator indicates that the voltage generated at the current detection resistance has exceeded the second target signal. Having the logic circuit, the operation of the second latch is ensured, and thus malfunction of the motor driver can be reduced.
Preferably, the period of the reference pulse is roughly constant. With this configuration, the period of the timing at which the drive transistors are turned ON is made constant. This makes it easy to take a measure for reducing influence of noise generated by the switching.
Preferably, the torque signal generation circuit uses a voltage corresponding to the torque command signal as the first target signal, and generates a sawtooth wave having a period equal to the time period corresponding to the predetermined electrical angle and having a peak value roughly equal to the first target signal based on the position signal and the first target signal and uses the sawtooth wave as the second target signal. Having these signals, the waveform of the phase currents can be made roughly trapezoidal, and thus sharp change of the phase currents is avoided.
Preferably, the ON-period control section generates a signal for controlling the switching operation so that the switching element to be kept ON in the second period among the plurality of switching elements to be turned ON in the first period is kept OFF until a lapse of a predetermined time after start of the first period, and outputs the signal as the switching control signal. This prevents currents of two phases from starting to flow simultaneously, and thus influence of switching noise is suppressed.
The present invention is also directed to a motor drive method for a motor driver having a plurality of output circuits each including an upper side switching element and a lower side switching element connected in series, and a current detection resistance connected in series with the plurality of output circuits in common for detecting a current supplied to the plurality of output circuits, the motor driver supplying a current to a motor from a connection point between the upper side switching element and the lower side switching element of each output circuit. The motor drive method includes the steps of: determining a position signal corresponding to the position of a rotor of the motor; selecting one switching element of one of the plurality of output circuits according to the position signal and turning ON the selected switching element for a time period corresponding to a predetermined electrical angle; and switching lower side switching elements of a plurality of output circuits among the remaining ones of the plurality of output circuits when the selected switching element is an upper side switching element while switching upper side switching elements of a plurality of output circuits among the remaining ones of the plurality of output circuits when the selected switching element is a lower side switching element, the switching operation being controlled according to an input torque command signal and a voltage generated at the current detection resistance so that each of a plurality of periods obtained by dividing the time period corresponding to the predetermined electrical angle includes a first period in which a plurality of switching elements among the switching elements to be switched are turned ON and a second period in which one of the plurality of switching elements turned ON in the first period is kept ON.